New Orleans , Louisiana -LRB- CNN -RRB- -- BP has put off crucial `` integrity tests '' on the new stacking cap on its ruptured Gulf oil well so it can review testing procedures , a review expected to take place through the night and into Wednesday .

The oil giant had expected the tests -- to check pressure in the well and determine if it can be sealed once and for all -- to get under way Tuesday afternoon .

But late Tuesday night , BP announced that additional analysis of the well testing procedure was needed . The move following a meeting with Energy Secretary Steven Chu and his team of advisers .

A source informed about BP operations told CNN 's John King that , `` There were some potential complications that might cause a delay -- some bad , some in the better to be safe than sorry category . ''

It had been hoped that the integrity tests would show whether an end is in sight to the environmental disaster that has been unfolding for the last 12 weeks . But throughout the evening , cameras some 5,000 feet below the surface showed oil gushing from the well 's capping stack , indicating that valves had not been closed to begin the pressure tests .

The massive custom-designed cap , which has a better seal than the last cap placed on the well , is some 30 feet high and weighs 160,000 pounds . It 's hoped that it might seal the well completely . But if it 's unable to contain all the oil , some could be diverted through riser pipes to ships on the surface . Under a worst-case scenario , however , tests might show there 's more damage to the well 's casing , meaning that capping the well would not stop the oil from flowing .

Retired Adm. Thad Allen , who is leading the federal response to the oil disaster , says the well cap placement is part a `` very complex , nuanced and broad-based response '' to the rupture of the underwater well in April , an accident that killed 11 oil workers and caused worst environmental disaster in U.S. history .

When they do take place , the integrity tests will measure pressure inside the well and will last anywhere from six to 48 hours . They will involve incrementally closing three valves on the new cap , a process that would allow BP to do its pressure measurements .

Higher pressure readings would mean the leak is being stopped , while lower pressure indications would mean oil is escaping from other parts of the well .

`` In this exercise , high pressure is good , '' Allen said . `` We are looking for somewhere between eight and nine thousand psi -LRB- pounds per square inch -RRB- inside the capping stack , which would indicate to us that the hydrocarbons are being forced up and the well bore 's being able to withstand that pressure . ''

Allen was asked what he thought the odds were to the success of being able to shut the well with the new cap .

`` I think we are very confident we can take control of this hydrocarbon stream and then slowly close all these valves and stop the emission of hydrocarbons . What we ca n't tell is the current condition of the well bore below the sea floor and the implication of the pressure readings , '' he said . `` That is in fact why we 're doing a well integrity test . ''

The latest containment cap , while seeming to offer the best odds of success , was n't ready earlier . In addition , BP has learned from previous , unsuccessful containment efforts , CNN 's Ed Lavandera reported on `` The Situation Room . ''

Allen said that if low pressure readings persist for around a six-hour time frame , that could signal problems with the new cap

If it 's determined that the cap ca n't seal the well completely , and some crude must be sent to the surface , the oil-gathering ship , the Helix Producer , in now in place . On Monday it joined the Q4000 , which was already active . And more vessels are planned . Allen said a four-vessel system that could recover 80,000 barrels a day could be ready by the end of the month .

Meanwhile , in another development Tuesday , the Obama administration sent BP and other responsible parties a fourth bill relating to the oil spill . The new bill is for $ 99.7 million . It comes on top of a total of $ 122.3 million in the first three bills .

The government says the parties are responsible for all costs associated with the spill , including stopping the leak , protecting the shoreline and long-term recovery efforts . The money will help replenish a $ 1.5 billion federal trust fund established to pay for damages associated with oil spills .

Among those costs is skimming oil on the surface of the Gulf . Allen said authorities are on pace to have around 1,000 skimmers available by the end of the month . At this point , there are fewer than 600 .

Work also has been continuing on the ultimate solution to fixing the problem , drilling two relief wells . But BP does n't expect the first relief well work to be completed until August .

Scientists estimate that 35,000 to 60,000 barrels of oil have spewed daily from BP 's breached well and Allen cautions that even if the engineering containment efforts work , there is still a lot to be done in a disaster that has affected the environment and the livelihoods of people from Louisiana to Florida .

`` There 's still a significant amount of oil out there , and the oil recovery and the impacts of this oil will probably extend well into the fall in terms of oil coming ashore , tar balls , beach cleanup , and then we will be moving of course at that point of the natural resources damage assessment trying to understand the long-term environmental ecological impact of the event , '' Allen said .

And the presidential commission tasked with investigating the Gulf oil gusher and making recommendations about the future of offshore drilling continued its public meetings Tuesday . The National Oil Spill Commission has six months to determine what happened when the Deepwater Horizon oil rig exploded April 20 -- and how to prevent something similar from ever happening again .

A new moratorium on deepwater drilling issued by the U.S. Interior Department Monday has already played a prominent role in the hearings .

CNN 's Campbell Brown contributed to this report .

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NEW : BP puts off crucial `` integrity '' tests to check effectiveness of new containment cap

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NEW : Testing procedures are being reviewed , a process expected to go through the night

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NEW : Move follows meeting with Energy Secretary Chu

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Thad Allen ca n't put odds on success of the new cap